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We report a new approach to produce high quality epitaxial graphene based on the concept of controlling Si sublimation rate from SiC surface. By putting a mask substrate to suppress Si sublimation from the SiC surface in ultrahigh vacuum, epitaxial graphene growth at 4H-SiC (0001) was locally controlled. Spatially graded surface graphitization was confirmed in a scanning electron microscopy contrast from the outside unmasked region to the inside masked region. The contrast was discussed with Raman characterization as the increase of graphene thickness and the surface compositional change of SiC. Results indicate two types of growth processes of epitaxial graphene at 4H-SiC (0001) step-terrace structures.

We investigated the temperature dependence (1200–2100 °C) of morphological features in the growth of epitaxial graphene on 4H-SiC (0 0 0 −1) C-face in ultra high vacuum (UHV). As a SiC initial surface preparation technique prior to the epitaxial graphene growth, Si-vapor etching was conducted to ensure a uniform step-terrace structure at 4H-SiC (0 0 0 −1) consisting of 0.5 nm step height. The epitaxial graphene growth has been performed in our original experimental setup with a TaC container inside an all metal chamber with radiative heating system, which allows to increase the graphitization temperature up to 2100 °C in UHV. The surface morphology of the epitaxially grown multilayer graphene, characterized by atomic force microscopy (AFM), exhibited a drastic change as a function of temperature. Ridge structures of the topmost graphene layer(s) appeared above 1600 °C, where the well-ordered step-terrace initial structure of the SiC vanished. In this high temperature range, the network of the ridges spread over the surface with an increase in temperature. Raman topography mapping for characterizing site dependent 2D-spectrum implies that rotational stacking disorder of graphene layers is accompanied by relatively small ridge network on the surface in a specific temperature range.

Surface graphitization of 4H-SiC (0001) at high temperature up to 2100 °C in ultra high vacuum (UHV) was observed for the first time in this study. From precisely controlled SiC step-terrace surfaces, uniform graphitized surface morphologies with large terraces appeared at anneal temperatures above 2000 °C. All the high-temperature processes were performed by using a newly constructed all metal chamber which enables to elevate temperature from 800 °C to 2000 °C within a minute in UHV. Initial step-terrace surfaces consisting of 1.0nm step height were achieved on 4H-SiC (0001) by utilizing newly proposed Si-vapor etching in a “semi-closed” TaC container. By estimations of Raman spectra, mainly bilayer graphene was obtained at temperatures from 1600 °C to 2100 °C. Estimation of Raman spectra also indicated that larger domain size graphene grows at higher temperatures, which coincides well with the results of morphological changes observed by AFM measurements. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The epitaxial graphene growth at the 4H-SiC(0001) surface with intentionally inserted step-free basal plane regions was performed by high temperature annealing in the range of 1600–1900 °C under ultrahigh vacuum. For fabricating inverted-mesa structures with the step-free regions at SiC surfaces, a combined process consisting of a direct laser digging and a Si-vapor etching at 1900 °C was utilized. The graphitized surfaces were characterized by atomic force microscopy, low acceleration voltage (0.1–1.0 kV) scanning electron microscopy and Raman spectroscopy. It was found that the graphene thickness at the SiC step-free surface tends to be suppressed compared with the thickness at background SiC step-terrace surfaces where the steps are intrinsically introduced from intentional/unintentional substrate miscut angles. From the characterization by Raman mapping, 1 ML graphene was obtained at the SiC step-free surface at 1600 °C graphitization in contrast to the case that multilayer graphene was grown at SiC step-terrace surfaces.